High speed mechanical, electro-mechanical flywheels and motor-generators are increasingly being used as an alternative to battery energy storage in both static and mobile situations due to their design longevity and capacity for handling very high energy charge and discharge rates. However, at working speeds, typically in the range of 20,000 to 120,000 RPM, these flywheels store very significant amounts of kinetic energy. If a flywheel were to fail, the instantaneous release of this energy poses a lethal risk to persons and the integrity of surrounding equipment through the release of ballistic fragmentation of the primary rotating flywheel fragments and a secondary field of fragments if the containment casing breaks up in response.
A number of energy storage flywheel containments are known which aim to provide a solution for the risks of mechanical failure of an energy storage flywheel. In US2005/0188777 for instance, a containment assembly is disclosed with a number of individual and coaxial cylindrical steel shields with a vibration damping material between the shields. The containment structure disclosed in U.S. Pat. No. 6,203,924 consists of a number of coaxial cylindrical layers with an inner structural layer, an energy absorbing layer and an outer support layer, wherein different materials are used for the successive layers.
The most common form of burst containment is that used in gas turbines, where expensive aerospace grade materials are used to form strong and discrete ‘hoops’ of energy-absorption material around specific narrow turbine blade discs. There may be several of these in the form of narrow annular layers down the length of a typical gas turbine engine.
Current containment solutions developed for mobile/automotive applications are typically cylinders made using machine-finished cast aluminium or are machined to final shape using chip removal from solid steel billet. These two solutions can be heavy, and in all known cases are bespoke-manufactured—making them expensive and too slow to make to be mass-production viable. Furthermore, the release of kinetic energy stored in a flywheel, which is in the order of 200 kJ for a small braking energy recovery system on a passenger car, would be expected to fracture and disintegrate such a containment vessels, resulting in a secondary and possibly more dangerous fragment field flying away from the device in unspecified directions, and at high speed.